The present invention relates to a boat for holding semiconductor wafers to be vertically stacked with gaps therebetween and to a heat treating system using the particular boat.
In the manufacture of ICs (Integrated Circuits), steps for subjecting a surface of a semiconductor wafer to a heat treatment for performing deposition or oxidation/diffusion, and steps for etching a film formed on the wafer are repeated several times. In applying a heat treatment to the wafer, a vertical heat treating system of a batch-processing type is generally used because a large number of wafers can be subjected to the heat treatment simultaneously. The vertical heat treating system includes a heating furnace having a process chamber made of quartz with a ceiling, and a heater arranged along the outer circumferential surface. A wafer boat holding a large number of wafers to be vertically stacked with gaps therebetween is arranged within the process chamber. Under this condition, a process gas is supplied into the process chamber to set up a process pressure and the wafers are maintained at a process temperature so as to carry out the heat treatment.
FIG. 14 is a perspective view showing a conventional wafer boat 1 together with a transfer device 14 and a boat elevator 12.
As shown in FIG. 14, the wafer boat 1 comprises four support rods 6A, 6B, 6C, and 6D. The upper and lower ends of these support rods are fixed to end plates 2 and 4, respectively. A large number of grooves 8A, 8B, 8C, and 8D are formed at a predetermined pitch on the circumferential surfaces of these support rods 6A, 6B, 6C, and 6D, respectively. A horizontal wafer supporting level is defined by these four grooves 8A to 8D having the corresponding height. To be more specific, the lower surface at the peripheral portion of a wafer W is allowed to abut against the four grooves 8A to 8D for the wafer W to be held on the wafer boat.
The wafer boat 1 is disposed on a heat insulating cylinder 10 made of quartz, which is moved up and down by boat elevator 12 having, for example, a ball screw arranged therein. The transfer device 14 movable both vertically and horizontally for the transfer of wafers W into and out of the wafer boat 1 is arranged adjacent to the boat elevator 12. The transfer device 14 includes a transfer arm 16 for holding a wafer W horizontal, the transfer arm 16 being capable of a reciprocating movement.
Where a wafer W is transferred into the wafer boat 1, the transfer arm 16 holding the wafer W is moved toward the wafer boat 1, as shown in FIG. 14. Then, the peripheral portion of the wafer W is positioned in the grooves 8A to 8D as shown in FIG. 15. Under this state, the transfer arm 16 is slightly moved downward. As a result, the wafer W is held in the grooves 8A to 8D so as to transfer the wafer W from the transfer arm 16 onto the wafer boat 1. Then, the transfer arm 16 is moved backward so as to be withdrawn from the wafer boat 1. The operation described above is repeated many times to cause a large number of wafers W to be held at a predetermined pitch by the wafer boat 1.
The wafer boat 1 holding these many wafers W is moved upward by the boat elevator 12 so as to be loaded into the process chamber (not shown) from the lower end portion of the process chamber. These many wafers held at a predetermined pitch by the wafer boat 1 are subjected to a heat treatment within the process chamber. Where the wafers W are unloaded from the process chamber, the wafer boat 1 is moved downward by the operation opposite to that described previously.
However, it has been found that, in the case of using the conventional wafer boat 1, a large number of particles are generated when the wafer boat 1 is loaded into or unloaded from the process chamber and when the wafers W are transferred into or out of the wafer boat 1, leading to a decrease in the product yield.